Monocrystalline materials offer a high degree of uniformity in terms of their physical properties as well as a high degree of repeatability and efficiency when compared to polycrystals of the same material. Therefore, monocrystals or single crystals are highly prized as replacements for polycrystalline materials, wherever such materials are used. Moreover, because of their highly desirable properties, monocrystalline materials are sought out for applications for which polycrystalline material would never be considered. For example, in optical applications, polycrystals will provide a diffuse reflection of incident light whereas monocrystals would yield specular reflection.
In accordance with the present invention, and indeed generally, monocrystals can be differentiated from polycrystals based on a number of factors. Monocrystals are sized and shaped such that they can be used individually in the production of sensors, probes and the like. Polycrystalline materials are those made up of a composite of many individual crystals. Many ceramic materials are polycrystalline in nature as are many rocks and fabricated metals. The size of these polycrystals are usually small with equivalent diameters in many materials varying from a few micrometers to about 100 micrometers. Much larger crystallites are possible. However, because of physical properties such as, for example, packing density and other issues common in the ceramics field, it is usually advantageous to insure that the individual crystals do not get too large and are of as uniform a size and composition as possible. In the context of the present invention, however, when referring to a single crystal, preferably, an isolated crystal which is large enough to manipulate i.e. sliced or shaped, is envisioned. This allows one to take advantage of the properties of same with respect to the individual orientation of the crystal. In preferred embodiment, individual crystals are about 100 microns in size or greater and more preferably, at least about a millimeter in size or greater, along one edge. Certainly, crystals of a centimeter on a single edge would be considered rather large.
There are other significant differences as well, most of which stem directly from the fact that monocrystals are, as their name implies, singular, whereas polycrystals involve the interaction of at least two crystals and suffer from charge carrier scattering at their grain boundaries, where modifications to the electric conductivity likely takes place. In fact, transport processes such as resistivity and dielectric properties of the crystal differ considerably from those of ceramic samples. Polycrystalline materials can be thought of as a composite material made up of two or more distinct individual components. Just as the polycrystalline materials are composites of the individual crystals, so too are their properties. Polycrystals have voids and often certain other stoichiometry and phases. These features each have an effect on the overall properties of the material and any device or sensor made using them. For example, the dielectric tensor of semiconducting polycrystalline materials is less than that of the corresponding semiconducting monocrystal of that same material because of the presence of voids. The resistivity of the polycrystalline material is also affected thereby. Monocrystals, which do not suffer from such composite properties will not exhibit such a strong dispersion in their impedance-frequency characteristics.
Moreover, since no two groups of polycrystals can be exactly the same, i.e. same number of crystals of identical size, orientation, stoichiometry and composite properties, the response of one sensor made with one group of polycrystals may vary with respect to other such sensors. Polycrystals may also be problematic because they may absorb water, particularly in the voids between crystals. When such material is exposed to variations in humidity, "aging" or a lack of reproducibility of properties over temperature may be accelerated in comparison to comparable monocrystals. Moreover, the size of the voids between individual polycrystals may change with time and exposure to the elements and in response to external electric fields. Again, the thermal and electrical properties of the resulting material may therefore change over time. Monocrystals do not suffer from these same aging limitations.
Finally, with regard to certain materials and in particular nickel-manganese-oxide cubic spinel crystals, there may be magnetic ordering effects at lower temperatures in monocrystals. But with polycrystals, exposure to a magnetic field may cause movement of the individual crystals. This would result in a change in the grain boundaries and the size and shape of any voids and lead to hysteretic effects (non-reproducibility). The composite properties of the material would change accordingly. Certainly, the magnetic, thermal and electric properties of monocrystals can be more accurately measured and more repeatedly relied upon than polycrystalline materials.
Monocrystals of nickel-manganese-oxide cubic spinel are particularly desirable. While they would not suffer from the disadvantages of polycrystals, they would exhibit all of the advantageous properties polycrystals of the same material. It is known, for example, that nickel-manganese-oxide cubic spinel polycrystals exhibit a very variable electric conductivity over a generally small temperature intervals such as, -55.degree. C. through 125.degree. C. With crystal doping, these ranges can be modified and/or extended in either direction. Thermistors made of monocrystalline nickel-manganese-oxide cubic spinel crystals, therefore, would be particularly desirable for use in sensors, thermometers and temperature responsive circuits.
Cubic spinel crystals such as the crystals of the present invention, provide isotropic properties when compared to crystals of other geometric configurations. For example, transport processes, such as electric conductivity, are isotropic.
Polycrystals of nickel-manganese-oxide have been reported by a number of investigators and, in particular Dr. D. G. Wickham, et al. .sup.(1)(2) and their properties have been studied and documented. Dr. Wickham's work, along with his collaborators, illustrated and characterized many of the advantageous properties of cubic spinel nickel- manganese oxide polycrystals and also provided phase diagrams for the system.
Nickel manganate monocrystals have, at least allegedly, been produced by at least two prior investigators. In "Growth of Nickel Manganate Mono-Crystals",.sup.(4) H. Makram reported the growth of NiMn.sub.2 O.sub.4 crystals having a ratio of two parts manganese to one part nickel using a flux method. The crystals were described by Makram as being sound and flawless and having linear dimensions of up to six millimeters. Crystals were grown in a flux of bismuth and boron oxides. The composition was described as being 56.7 Mol % Bi.sub.2 O.sub.3, 13.3 Mol % B.sub.2 O.sub.3, 30 Mol % NiO and 30 Mol % MnO.sub.1.365.
The Makram paper did not, however, provide sufficient information by which the nature and composition of the resulting crystals could be identified and/or verified. Neither a Curie point nor lattice constants were provided. As it is not uncommon for various crystal forms to look like other crystal forms without the provision of more objective data, there are reasons to be concerned as to whether or not the results reported were accurate with respect to the 2 to 1 manganese to nickel stoichiometry. Upon a more detailed examination of the Makram paper, several other critical problems were revealed. These problems made it impossible, even for those of extraordinary skill in this art, to reproduce the work. In fact, the inventors have never been able to exactly reproduce the Makram result.
One example of the problems caused by various disclosures within the Makram paper is that the "typical formulation" reported yields 130 Mol %; a number which is clearly impossible. Makram also required the use of an unquantified amount of an unknown formulation i.e. MnO.sub.1.365. As a wide variety of crystalline states may result if the correct elemental ratios are not maintained, this proved to be a very troubling development. Unfortunately, there was no way to determine what actual ratios should be used based on the Makram paper.
Makram also suggested the use of H.sub.2 O or air for quenching. However, the use of oxygen species could effect the chemistry of the resulting crystal. Finally, while Makram suggested that the resulting crystals had a 2 to 1 ratio of manganese to nickel, when the inventors attempted to reproduce the work, ratios of closer to about 3 to 1 were observed.
The other investigators who allegedly produced a monocrystal of nickel manganate (NiMn.sub.2 O.sub.4) were V. A. M. Brabers and J. C. J. M Terhell..sup.(5) Brabers et al. used a chemical transport method to produce various crystals. However, again, no lattice parameter was given. The ratio of manganese to nickel given for the allegedly resulting crystal was about 2.1:1. Because of the complexity of the process and its scale, the process was not considered practical.